ESD protection structure using LDMOS diodes with thick copper interconnect

ABSTRACT

An interconnection structure and method for a multiple zener diode ESD protectoin circuit for power semiconductor devices. A plurality of lateral Zener diodes is formed. Each device is formed of a plurality of cathode and anode diffusion regions to be coupled together to form the cathode and anode of one or more Zener diodes. Each diffusion region has a first metal layer stripe formed over it and in electrical contact with it. A second metal layer conductor is formed over a plurality of the first metal layer stripes, and selectively contacts the first metal layer stripes to form a bus. A thick third metal layer is then formed over each second metal layer bus, either physically contacting it or selectively electrically contacting it. The thick third level metal is fabricated of a highly conductive material, such as copper. The resulting Zener diodes are coupled together in an ESD structure using the second level busses and the thick copper third level busses. The ESD structure of the preferred embodiment has low overall resistance and fast time to breakdown and provides excellent protection for the circuits to be protected. Other devices, systems and methods are also disclosed.

FIELD OF THE INVENTION

This invention relates generally to programmable integrated circuitsutilizing lateral double diffused MOS technology, to the fabrication ofLDMOS transistors, to electrostatic discharge (hereinafter ESD)protection circuits for LDMOS transistors and specifically to theapplication of linear geometry lateral DMOS cells and circuits totransistors and integrated circuits for power applications, circuits andsystems.

CROSS REFERENCE TO RELATED APPLICATIONS

This application relates to co-pending patent applications:

TI-17968, entitled "Semiconductor Device Having Thick CopperMetallization", filed Aug. 31, 1994, U.S. patent Ser. No. 08/299,177;

TI-16545, entitled "A Method for Current Ballasting and Busing overActive Device Area Using a Multi-Level Conductor Process", filed Mar.13, 1992, U.S. patent Ser. No. 07/850,601;

TI-17846, entitled "LDMOS Transistor with Thick Copper Interconnect",filed Nov. 2, 1994, U.S. patent Ser. No. 08/333,461; and

TI-20030, entitled "A Multiple Transistor Integrated Circuit with ThickCopper Interconnect", filed Nov. 2, 1994, U.S. patent Ser. No.08/333,174;

each of which is assigned to Texas Instruments Incorporated.

BACKGROUND OF THE INVENTION

In producing integrated circuits for power applications it is typical touse a process utilizing a lateral double diffused MOS (hereinafterLDMOS) technology. Devices are often designed using a plurality oflateral diffusions which are then coupled together to make a singlelarge current capacity device.

In the prior art, single and double level metallization schemes are usedto make the connections between the various diffusions and thereby formthe large devices required for power circuits. Because the lengths ofthe metal runs are quite long, current distribution tends to becomenonuniform across the devices. As a further consequence, debiasing alsooccurs along the metal lengths, which means different areas of thedevice are operating at different potentials. Non-uniform operation ofthe device results from this metal debiasing and the various diffusedareas switch at different instants causing nonuniform currentdistribution.

Although the conventional two level metallization schemes of the priorart can be optimized to reduce the current debiasing problems, forfabricating large devices carrying large current loads the problems withdebiasing remain. One approach to a two metal interconnection scheme foran LDMOS power device composed of many diffusion stripes is described ina co-pending U.S. patent application entitled "A Method for CurrentBallasting and Busing over Active Device Area Using a Multi-LevelConductor Process", TI-16545, U.S. application Ser. No. 07/850,601,assigned to Texas Instruments Incorporated. Although the techniques andstructures for placing the contacts and vias in a two level metalinterconnection scheme described in this earlier patent application willreduce the debiasing effects as much as possible using conventionalmetallization techniques, the problems persist for large transistorshaving long interconnect metal lengths.

In a LDMOS device formed from a plurality of diffusions coupled togetherusing first and second levels of metal, the source and drain diffusionsare alternating stripes. The source and drain diffusions are coveredwith, and in electrical contact with, stripes of the first metal layer,which is typically aluminum having a thickness of up to 1 micron. Thefirst level metal is then covered with an isolation oxide. The secondlevel metal is then used to form source and drain buss lines, eachrunning over many of the source and drain diffusions, and eachselectively coupling many of the first level metal stripes to a singlebus through the use of contacts cut through the isolation oxide. Thissecond level of metal can have a thickness of up to 3 to 4 microns. Thissystem of interconnect is completely described in the active areabussing patent.

In the prior art LDMOS structures, the second level of metal looks likea resistor in series with the source or drain bond pad and the paralleldevices. The amount of resistance provided by the metal interconnectionis critical to the performance of the device because the criticalparameter for performance, Rdson, is directly proportional to thisresistance. For optimal performance of the completed device it istherefore desirable to minimize the metal resistance.

Modeling techniques have shown that for an exemplary LDMOS transistorcomprised of 11 paralleled sections, each section having up to 150diffusion stripes, coupled together using standard 1 micron first levelmetal and 3 micron aluminum for metal two in a conventional metal systemwith thicknesses as mentioned, the metal component of the critical Rdsonresistance is as great as 63% of the total Rdson. This 63% contributionto the Rdson resistance is from the metal itself and the debiasingeffects caused by the metal. Because of the metal resistance a largearea of silicon is needed to lower the total Rdson of the device.

Other problems with the conventional methodology are also significant.Because the aluminum metallization scheme provides a somewhat resistivepath for the current running along the diffusion stripes, there is anincrease in the source voltage as measured from the end closest to thesource bus to the other end of the diffusion. In an LDMOS transistorstructure, this debiasing effect is of great concern because of theimportance of the critical voltage Vgs. As the source voltage increasesalong the metal run, the voltage Vgs is reduced. As a result, in areasfarther from the source pad there is nonuniform operation. As the sourcepotential rises for a given gate voltage Vg, Vgs drops, the transistorsegments receive less drive, and the overall device Rdson increases. Thenon-uniform current distribution due to non-uniform operation of thesections of the LDMOS device also leads to safe operating area problems,in the form of reducing the safe operating area of the device. In acondition where the gate voltage Vg is low, these problems become evengreater, and device operation becomes marginal earlier than desiredbecause the effective voltage Vgs is being significantly reduced inregions of high source debiasing. Drain debiasing is a problem also inthat the drain potential drops across the device and the availabledesign drive potential is not evenly distributed.

Additional problems arise due to the electromigration current densityrules required with conventional metallization systems. Each of thesections of the LDMOS device is covered with a second level bus for thesource and another for the drain. These busses are coupled together atthe ends of the device. To meet the safe operating requirements usingconventional metallization schemes, the busses at the ends of the devicehave to be made wider and wider as more sections are added, consumingproportionally more and more non-active device area to achieve largerdevices.

Further problems arise as higher current capacity devices are designedusing the prior art techniques. The nonuniformity of currentdistribution associated with debiasing can lead to so called "hotspots", areas where localized current exceeds the thermal power limitsof the device, and premature failure locations are the result. Thesepremature failures further result in lower peak current capacity ratingsfor the devices and a reduced safe operating area rating. A need for animproved method for designing lateral power devices which enhancesuniform current distribution and device operating efficiency,eliminating current crowding and electromigration concerns, andproviding reduced Rdson performance, thus exists.

SUMMARY OF THE INVENTION

Generally, and in one form of the invention, a structure and method fordesigning a high current capacity ESD circuit is provided using alateral DMOS process, the device having a plurality of cathode and anodediffusions which are rectilinear in shape and laid out in rows. Theindividual cathode and anode diffusions are then covered with a firstlevel metal using a conventional metal layer, then a second level ofmetal is deposited perpendicular to the first level metal fingers. Thesecond level of metal forms cathode and anode busses which makeselective contact to the plurality of source and drain diffusions usingvias. A third level of metal is then deposited over the second levelmetal pattern using a thick conductive layer to form shorting busses.This third level is a low resistance material such as copper and isplated onto the device. The third level of metal is thick enough togreatly reduce the resistance of the metallization pattern and therebyreduce the Rdson resistance of the device. Further, current distributionand debiasing problems are greatly reduced. The invention isincorporated into a three diode ESD protection circuit. The third levelof metal is non critical to alignment and acts as very low resistancecurrent busses on the die.

A first advantage of the invention is that the use of the preferredembodiment results in a device with high current carrying capacity and alow Rdson device for a given area, and a greatly improved safe operatingarea characteristic. Another advantage is that the use of the preferredembodiment along the gate busses of the device reduces the R-C timeconstant during switching and reduces the switching loss. The use of thepreferred embodiment results in a faster switching time and helps toprovide uniform current switching at high operating speeds.

BRIEF DESCRIPTION OF THE DRAWINGS

In the drawings:

FIG. 1 depicts a source and drain area of a portion of a RESURF LDMOSdevice, showing the three levels of metallization in cross section;

FIGS. 2-4 depict the steps used to build the cross section of the deviceof FIG. 1;

FIG. 5 depicts a plan view of an LDMOS transistor using stripeddiffusion sections as shown in FIG. 1 and a first preferred embodimentof the invention;

FIG. 6 depicts a plan view of an LDMOS transistor using the stripeddiffusion sections as shown in FIG. 1 and a second preferred embodimentof the invention;

FIGS. 7-9 are plots of the debiasing characteristics for LDMOStransistors;

FIG. 10 is a table which presents the Rdson data for the devices shownin FIGS. 1-9;

FIGS. 11-14 depict a plan view for the different metallization layersfor producing a multiple transistor integrated circuit using a secondpreferred embodiment of the invention;

FIG. 15 depicts a plot comparing the Rdson resistance of a transistor ofthe embodiment of FIGS. 14-17 and contrasting the Rdson resulting fromthe advantageous use of the invention to the Rdson resistance of asimilar device fabricated using conventional layout techniques of theprior art;

FIG. 16 depicts a cross sectional and schematic view of a three diodeESD protection circuit incorporating the thick metal interconnect of theinvention;

FIG. 17 depicts a detailed cross section of one of the three diodes ofthe ESD protection circuit of FIG. 16; and

FIG. 18 depicts a plan view of the three diode ESD protection circuit ofFIG. 16.

Corresponding numerals and symbols in the different figures refer tocorresponding parts unless otherwise indicated.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

In constructing high power lateral devices using DMOS technology, manylateral drain and source diffusion regions are created and then coupledtogether. In operation, it has been discovered that the prior artlateral power devices tend to fail primarily due to abnormal nonuniformswitching in the long striped geometries used to form the source anddrain regions. The nonuniformity of switching appears to occur in partdue to the variation in current distribution along the striped sourceand drain diffusion lengths and the geometry of the metallization of thedevices.

In order to describe the operation and advantages of the invention, itwill be described in relation to an exemplary transistor implementedusing a lateral double diffused RESURF MOS process. This embodiment isintended only to illustrate, and does not limit, the method andstructure of the invention, and the advantageous use of the invention isnot limited to transistors, or to LDMOS processes. The use of theinvention will provide advantages with any process or structure wheremultiple regions are coupled together to form a single device, includingtransistors, resistors, diodes, capacitors, or other semiconductordevices.

FIG. 1 depicts a cross section of a lateral DMOS transistor implementedas a lateral device having a plurality of source and drain diffusionswith gate oxide and gate polysilicon running between the source anddrain striped diffusions. This transistor could be manufactured, forexample, using the lateral DMOS process as described in U.S. Pat. No.5,272,098, entitled "Vertical and Lateral Insulated Gate Field EffectTransistors, Systems and Methods", assigned to Texas InstrumentsIncorporated, and herein incorporated by reference. Alternatively, themethods of U.S. Pat. No. 5,242,841, entitled "Method of Making LDMOSTransistor with Self-Aligned Source/Backgate and Photo-Aligned Gate",assigned to Texas Instruments Incorporated and herein incorporated byreference, may be used. U.S. Pat. No. 5,306,652, entitled "LateralDouble Diffused Insulated Gate Field Effect Transistor and FabricationProcess", and also assigned to Texas Instruments, Incorporated andherein incorporated by reference describes the manufacturing of LDMOStransistors and more specifically reduced surface field effect (RESURF)transistors. Other methods of manufacture of lateral devices could beused as well.

FIG. 1 depicts a three dimensional view of a portion of the finishedtransistor. Silicon substrate 1 is shown with an epitaxial layer 3deposited over the substrate. A p type diffusion well 5 is formed usinga dopant implant and a diffusion step. Polysilicon gate 15 is depositedto complete the gate formation for the transistor. Source and drainregions 9 are formed using a second dopant implant, as is backgatecontacting region 11, which is the same conductivity type as thediffusion well, but a heavier concentration. The backgate and source anddrain regions are then completed using a second diffusion step. Theseimplants are done using the gate polysilicon 15 in conjunction with thedrain LOCOS region 13 as a mask, so that the gate and source and drainregions are self aligning. Isolation oxide is formed over the gate andgate oxide regions. A pattern and oxide etch is applied to create thecontacting regions in regions 9 and 11 for connecting the metallizationlayers to the silicon surface. A first metallization layer 17 isdeposited and patterned to contact with the source and backgate regions,and also in contact with the drain regions. These first metal regionsare electrically isolated, that is they follow the source and drainregions and are not coupled together.

A second isolation oxide 19 is deposited over the first metallizationlayer. Vias are patterned and etched in this second isolation oxide inplaces where the first and second metallization layers are to beinterconnected. A second metallization layer 21 is then deposited tocomplete the structure. This second layer is used to selectivelyinterconnect multiple source or drain diffusion regions to form largesource and drain areas for the transistor. The third level metal 23 isthen deposited by electroplating over the second metallization layer 21.In this first embodiment, the second and third level metallizationlayers are shown as being in physical and electrical contact. However,in other regions, a protective overcoat is placed over the secondmetallization layer, and this protective overcoat is patterned andetched so that the third metallization layer is electrically isolatedfrom the other levels. Vias can again be used to couple the third levelof metal to the second level to reduce the debiasing effects of thefirst two level metallization structure.

FIGS. 2-4 depict in cross section the process steps used to fabricatethe transistor of FIG. 1. Source and drain regions are formed using thesteps as described in the patents as incorporated herein above. In FIG.2, the p type substrate is shown after a p type epitaxial layer 3 isformed overlying it. Epitaxial layer 3 may be formed by using well knownchemical vapor deposition techniques, molecular beam epitaxy or ionimplantation. Since in this application the epitaxial and substratelayers are of p type, boron is used as the dopant during the formationof layer 3. The gate oxide 13 and the polysilicon gates 15 are formed.The source and drain contacting regions 9, along with the bodycontacting regions 11, are formed to contact the diffusion well 5 forthe source, and resurf area 7 for the drain. The gate isolation oxide 14is a thin oxide which is deposited then patterned and etched to leavethe drain and source regions 9 and the backgate diffusions 11 exposed toreceive the first layer of metallization in electrical contact. Thefirst metal 17 layer is deposited to contact the source and draindiffusions 9 and the backgate diffusions 11. The first metal layer 17may be fabricated using well known metallization systems, such asaluminum, or a barrier metal and aluminum, and is formed using LPCVDtechniques. The first metal layer 17 has a thickness in the range ofapproximately 1 micron. Then, the second metallization layer 21 isformed. This layer selectively couples first metal which is in contactwith either source or drain diffusions. An isolation oxide 19 is formedover the entire substrate. This oxide is then patterned to expose theareas of the first metal layer 17 that are to be coupled to the secondmetal layer. The second metal layer 21 is then deposited and patternedover the substrate forming columns of source or drain striped regionsthat are coupled together by the vias in oxide 19. Second metal layer 21is formed over the oxide 19 and fills the vias to couple the secondmetal 21 to first metal layer 17, the second metal layer being formedusing the same technique as that used for the first metal layer, andbeing of a conventional material, such as aluminum with a maximumthickness in the range of 3-4 microns. This maximum is determined by thecapability of the conventional processing techniques.

FIG. 3 depicts the first steps used in fabricating the third levelcopper layer of the preferred embodiment. After the second metal layer21 is patterned a protective overcoat is applied to the wafer in theform of a deposited nitride layer as commonly used in semiconductorprocessing. The next steps is a patterning step which opens exposes theregions of metal two in the bond pad locations. A via pattern is alsoapplied which sets up vias to make contact between the third levelmetallization layer, not yet formed, and the metal two layer 21. Abarrier layer 20 of Ti--W is deposited over the entire substrate,protecting the second metal layer 21 and the underlying layers. Thislayer typically 1000 Angstroms thick. A seed layer 22 for a metalplating operation is then deposited onto the barrier layer. This seedlayer will be of the same type of material as the final third levelmetal layer, and in a preferred embodiment it will be of copper orcopper alloy. This layer must be sufficiently thick to support a platingoperation, and is typically 2000 Angstroms thick. Other usefulalternative materials for the third layer include other superiorconducting materials such as gold, however copper is a better conductorand therefore preferred over gold, and is also an inexpensive material.

FIG. 4 depicts the completed structure, and repeats FIG. 1. To completethe device, a thick layer of copper is plated over the seed layer 22using an electroless or electroplating technique. The thick layer ofcopper 23 is patterned using a negative photoresist mask, that is athick layer of photoresist is patterned and etched such that photoresistremains only where the thick metal three layer is not desired, then thecopper is plated using an electroplating technique on the seed layer,and the resulting copper bus is plated up to the height of the thickphotoresist or less if desired. Typically, the copper layer will beplated to a thickness of 15-35 microns. In order to prevent problems atthe top of the plated structure, the photoresist layer must be thickerthan the final copper layer. Thicker copper structures can be plated onfor even lower resistivity. The photoresist and the seed layer under thephotoresist are then stripped from the areas where no copper is platedusing conventional processing techniques. The barrier layer of Ti--Wprotects the bond pad areas where the third level of metal is notdesired from getting etched away while the copper seed layer is removed.The Ti--W layer is then removed using a selective etch. Finally, aplating step of an inert material is performed on the copper third levelmetal layer 23 using an electro or electroless plating process. In apreferred embodiment, this material is nickel, but other like materialscan be used, such as palladium. This step is optional but when used itis performed to prevent corrosion products and undesirable reactionswith the copper third level metal layer, and the copper aluminuminterface between copper third metal layer 23 and the second metal layer21. In addition the nickel forms an acceptable medium for aluminumbonding, if desired. Typically the third level metal layer will notextend to the bond pads, but in some alternative embodiments it couldextend to the bond pads. If used, the nickel coating is plated to athickness of about 1 micron using conventional techniques. The nickelcoating covers the top and may or may not cover the sides of the copperconductor, depending on the process.

The copper third level metal layer 23 of FIGS. 1 and 4 is shown inphysical contact with the second layer and at least partially overlyingit. That is, the copper third level metal layer is formed everywhere thesecond level metal is formed and in physical contact with it.Alternatively, if desired, the protective overcoat may be formed overthe entire second layer metal level 21 between the second metal and theTI--W and copper seed layers, and the third level metal may beselectively coupled to the second level periodically using vias throughthis protective overcoat to the second metal layer.

FIG. 5 depicts a plan view of a portion of a transistor fabricated usinga plurality of the LDMOS stripes as shown in cross section in FIGS. 1-4.A plurality of drain diffusion stripes 25 are shown alternating with aplurality of source diffusions 27. Gate polysilicon regions 30 are shownrunning in parallel with and between the source and drain diffusions.Gate connect buss 29 is shown coupling the gate polysilicon regions 30together to form the gate for a single LDMOS transistor. Source pad 31is shown at one end of the column of diffusion stripes. Drain pad 33 isshown at the other end of the column of diffusion stripes.

The second level metal layer is used to form source and drain busconductors 34 and 35. Where a source bus is desired, the second levelmetal layer source bus 34 is coupled through selective vias, not shownin the figure, to the first metal stripes which are in contact with thesource regions 27. Similarly, the drain bus is formed by selectivelycoupling drain regions 25 by running the second level metal layer drainbus 35 perpendicularly to the first level metal stripes, and selectivelycoupling the second level metal bus 35 to the first metal drain stripes25 using vias which are not visible in the figure. Finally, third metalshorting buss regions 39 and 41 are deposited over the second levelmetal source and drain bus regions, third level metal region 39overlying and in physical contact through vias 36 with the source bus34, and third level metal region 41 overlying and in physical contactwith the drain bus 35 through vias 36. Note that in the preferredembodiment, the third level metal of thick copper is used to short thesecond level metal busses together, and is not used over the bond pads.This allows the use of conventional bonding techniques, while stillgreatly lowering the overall resistance of the device.

In operation, the distribution of current through the first and secondlevel metal layers of the LDMOS transistor formed by coupling thestriped diffusions together in FIG. 5 is critically effected by thenumber of contacts and vias used, and their spacing. A technique tooptimize the contact and via placements for the first and second levelmetal layers has been disclosed in co-pending patent application titled"A Method for Current Ballasting and Busing Over Active Device AreaUsing a Multilevel Conductor Process", U.S. patent Ser. No. 07/850,601,filed Mar. 13, 1992, TI No. TI-16545, and assigned to Texas InstrumentsIncorporated. The technique of TI-16545 may be used with the thick thirdlevel metal interconnection technique used here, although other methodsof coupling the first and second layers are also compatible with thethick third level metal of the preferred embodiment described here. Thethick third level shorting buss regions of FIG. 5 greatly reduces the onresistance Rdson of the transistor, and also contributes to a uniformoperation of the device may reducing debiasing across the busses of thedevice.

In operation, the LDMOS transistor of FIGS. 1-4 will receive currentthrough it from the second level metal, which is coupled to the bondwires of the device at the bond pads. The incoming current will see astructure with very low resistance because the copper third level metalshorting buss is an excellent conductor and is of a significantthickness. Current will then be carried across the device and will flowinto the second level metal and into the first level metal stripes,where it will flow into the source diffusions. Assuming the gateconductors are on, that is the gate terminals have a positive potentialgreater than the threshold voltages, the current will flow out of thesource regions, across the gate regions, and into the drain diffusions.Again, the current will see a low resistance structure coming out of thedrains, as the copper third level metal shorting buss is a significantpart of the path from the drain diffusions to the second metal bond padand then to the bond wire for the drain.

An alternative embodiment to the arrangement of FIG. 5 is shown in FIG.6. Again, source diffusions 54 are formed in rows that alternate withthe drain diffusions 52. A first level metal layer is formed over andcoupled to the source and drain diffusions to form first level metalsource and drain stripes. The second level metal busses are runperpendicular to and over the first level stripes, so that a source buss53 and a drain buss 55 each run over and each partially cover eachsource or drain striped region. Again, the source and drain regions 54and 52 are selectively coupled to the second level source and drainbusses 53 and 55 using vias 58 and 56 from metal one up to metal two.There are a plurality of these source and drain busses 53 and 55 whichalternate and form columns across the device. Each column also has anassociated polysilicon gate bus 51 running along side the source anddrain busses, and coupled to polysilicon gates between the drain andsource stripes that are partially overlying the source diffusions, butare not shown in FIG. 9 for clarity. The third level metal shortingbusses 59 and 57 are deposited over the second level metal busses asbefore, however now the third level metal is running perpendicular tothe second level metal busses, and therefore in parallel with thediffusion rows and first metal stripes. These third level metal bussesselectively via down to the columns of second level metal to form largesource or drain shorting busses. Third level metal buss 59 is coupled tothe source columns 53 by vias 63. This bus is shown cutaway over thecolumn at the left hand side of FIG. 9 so that the source and draindiffusions and the second level to first level metal vias 58 and 56 canbe shown, but 59 is a continuous bus that runs all of the columns.Similarly, third level buss 57 is coupled to the drain columns of secondmetal 55 by vias 61. This structure is used for forming very largedevices with many hundreds of striped regions coupled together. Informing these devices, it is important that the third level metal beisolated from the second level, usually by a protective overcoat layer,and that the third level metal is perpendicular to the second level ofmetal and preferably parallel to the first level metal stripes. Ifadditional levels are used, each alternating level should beperpendicular to the levels just above and just below it.

Other embodiments where the copper is used to lower the resistance ofall or part of the interconnect structure for a lateral device are alsocontemplated. The thick third level metal shorting busses may be laiddirectly on the conventional second level metal, and in electrical andphysical contact with it, alternatively the copper may be isolated byprotective overcoat or oxide layers, and be coupled to the conventionalsecond level metal by means of selectively cutting vias through theisolation layer and forming vias to the second level metal layer asshown in FIGS. 5 and 6.

The significance of the three level metal interconnection system of thepreferred embodiment may be better understood by considering how currentdebiasing effects occur in the prior art lateral transistors and inlateral transistors using the preferred embodiment of the invention. Themetal interconnections used in a power device fabricated using multiplediffusions coupled together contributes a significant amount to theresistance of the device, to current debiasing and to changes in theuniform operation of the device. The metal interconnections alsoadversely increase the critical performance parameter, Rdson. Usingcircuit modeling techniques, it has been determined that for an LDMOStransistor having a conventional two level metallization interconnectscheme, the metallization contributes as much as 63% of the overall onresistance of the device for devices of less than 100 milliohms ofresistance. In contrast, a model of a transistor using the preferredembodiment of the invention disclosed herein and depicted in FIG. 5 hasbeen calculated to reduce the metal contribution to the device Rdsonresistance to 15% of the overall Rdson resistance. Further improvementsare seen in the uniformity of operation of the transistor. The criticalvoltage, Vds, should be uniform along the length of each stripe,eliminating the problems caused by nonuniform switching in the prior artdevices.

FIGS. 7-10 depict the advantages of the invention using modeling resultsfor an exemplary LDMOS transistor structure. The LDMOS transistorconsidered here comprises 200 striped diffusions similar to thosedepicted in FIG. 8. The pads are located at the ends of the column ofstriped diffusions. The stripes are 400 microns in length. FIG. 7depicts the voltage potential variations predicted along the secondmetal buss length using a conventional two level metal system with 3micron aluminum used as the second level metal layer to connect the 200striped geometry diffusions together.

In FIG. 7, the voltage differential is shown normalized to 1.0.Likewise, the distance along the second metal buss length is normalizedto 1.0. Ideally, the drain voltage Vd would be 1.0 at all points alongthe buss. The source voltage Vs would be 0.0, and the Vds would ideallyalso be 1.0. FIG. 7 depicts the results of the modeling simulation,assuming a typical silicon specific resistance Rsp of 0.82milliohms×cm². The curve marked out by rectangular points depicts thepotential Vs at the source, along the length of the buss. The rise insource voltage along the length of the buss is due to the currentdebiasing which occurs as the current flows down the metal buss to thecontacted location. The curve plotted with triangular points depicts thedrain voltage Vd along the buss length. The drain region demonstratesthe same debiasing phenomenon as the source region, but instead ofrising above zero, the drain region debiasing results in a voltage dropVd below the applied potential from the current flow in the buss. Thecurve in FIG. 7 marked with circles depicts the differential drain tosource voltage, Vds, along the buss. Ideally, Vds would be thedifference between the applied potentials at the contact points, that is1.0, along the length of the buss. FIG. 7 depicts the result ofdebiasing in a two level interconnect system on Vds. The voltage Vdsstarts at either end of the bus far lower than is desired, and in themiddle of the bus it falls even farther. The result will be a devicewith much poorer performance than desired, or expected.

FIG. 7 illustrates that in a large LDMOS device constructed frommultiple striped geometries connected together, a huge debiasing effectoccurs along the buss length. A major loss in performance is experiencedfrom the metal resistance added by the metal interconnection scheme andalso the lowered drive voltage Vgs that results from the debiasing risein the source voltage along the buss length.

FIG. 8 depicts, for contrast, the debiasing effect predicted by the sameelectrical model if only a single diffusion stripe of 400 microns inlength were tied to the shorting buss. This structure can only be usedfor a small transistor. Again, only first and second metallizationlayers of conventional materials and thicknesses are used. Thetriangular curve shown in FIG. 8 depicts the drain voltage as measuredalong the buss length. Clearly, no significant debiasing occurs, as thecurve for the drain voltage approaches the ideal, that is the entirelength of the buss the drain region is at the potential applied at thecontact point, here shown normalized to 1.0. Also, the source voltage Vsis plotted as the curve shown with rectangular data points. Again, nodebiasing occurs, and the source voltage also retains a fiat, almostideal shape, and remains at the 0 level along its entire length. Sinceboth the drain and source voltages show no debiasing, the differentialvoltage Vds can be expected to remain ideal as well, and remains at the1.0 level as shown by the curve plotted using the circular data pointsin FIG. 8. Comparing FIGS. 7 and 8, it can be seen that when largestructures are designed using the conventional two level metalinterconnection structures of the prior art, significant ill effectsoccur due to the debiasing and the resistance of these structures. Forsmall structures using a single stripe, no debiasing effects occurs. Theinterconnection structure of the prior art therefore causes debiasing,higher resistance and a significant loss of performance for the largerdevices.

FIG. 9 depicts the results and advantages obtained using the inventionin a preferred embodiment. Now the three level metal system of thepreferred embodiment depicted in FIGS. 1-6 is used. Two hundreddiffusion stripes, each 400 microns long are formed, and over theconventional first and second level metal layers, a third level of 25micron thick copper is used to connect the source and drain regions.Again the drain voltage is plotted as a curve using triangular datapoints, normalized to 1.0. At the contact point the voltage is 1.0, andas the data is collected along the buss from the right edge of the graphto the left, some debiasing does occur. However, in contrast to FIG. 8,the level of debiasing is greatly reduced and is limited to just over100 millivolts at the farthest point from the drain potential contactpoint. Similarly, the curve using the rectangles illustrates the sourcepotential Vs along the length of the buss. At the contact point thesource is at a normalized potential of zero, and rises as the data iscollected along the length of the buss. But at the farthest point, theright side of the plot, the source voltage is only 110 millivolts abovethe desired potential. The differential voltage Vds is kept close toideal by the lack of debiasing, and at its lowest point, in the middleof the buss, it drops about 180 millivolts. This is in contrast to thevery low Vds obtained using the prior art techniques, as shown in FIG.7.

FIG. 10 is a table that further illustrates the significance of the useof the preferred embodiment in reducing the active on resistance and thedebiasing effects. In FIG. 10 Column A shows the results obtained in theresistance measurements for the device of FIG. 7, using first and secondmetal layers only, and 200 stripes each 400 microns in length. Column Bshows the results of a single 400 micron stripe for comparison, thedevice simulated in FIG. 8, and illustrates the amount of resistancecontributed by the need to interconnect the stripes with metal to obtainlarger devices. Column C in the table of FIG. 10 illustrates theadvantages of the preferred embodiment. Now the third level of thickcopper is added to the conventional metallization scheme, greatlyreducing the resistance obtained for the device with 200 stripes.

A physical analysis can also be done that will give an idea of theexpected performance of the striped device. The silicon specific onresistance Rsp=0.82 milliohm×cm². Therefore, the expected on resistanceshould be the specific on resistance over the active area AA, or:##EQU1##

However, the electrical model illustrated in column A of FIG. 10indicates that for a 200 stripe LDMOS device using conventional metaltechniques to interconnect the devices, the equivalent circuit obtainedusing the model indicates that because of the effects of debiasing, theactual resistance Ron in the active area is Ractr1=0.2626 ohms, which isfour times the expected on resistance. Further, the model indicates that75.6% of this resistance is due to the metal for interconnecting thisstructure. Thus the debiasing effect causes the actual resistance in theactive area to be about 4 times the expected on resistance.

FIG. 10 in column B also indicates that for a single stripe device,requiring no metal interconnect, there is no debiasing effect. The Vd,Vs and Vds profiles of FIG. 8 present an ideal case for a small device,which is reflected in the Ron obtained in column B of FIG. 10.

The use of the invention in the striped device makes it possible toapproach the ideal case for the 200 stripe LDMOS device. The thirdcolumn, column C, of the table of FIG. 10 shows the advantage of the useof the invention. FIG. 10 in column C shows that the use of the copperthird level metal shorting busses coupled to the second level metalgives a real resistance of 0.0771 ohms, much closer to the ideal case ofcolumn B, and that the contribution of the metal interconnect to theresistance Ron is now only 16.9%.

FIGS. 11-14 depict, in a series of plan views, the metallizationinterconnect pattern from second metal through the third metal layer fora seven transistor circuit using the preferred embodiment shown above inFIGS. 1-5. The preferred embodiment described above shows a transistorfabricated having third level metal overlying large bussing regions butnot over the bond pad regions. The use of conventional second levelmetal to form the bond pad regions is advantageous because conventionalbonding systems are sufficiently low in resistance, and are cheaper thanbonding to the thick third level metal, which requires additional nickelplating of the third level metal. However, the lower resistance is stillrequired in the large bussing regions of the device to address thedebiasing problems described above. By using the copper buss barsoutside the bond pads, a device with a desired Rdson resistance may bemade in an area two to three times smaller than would be required usingthe conventional two level metallization structures of the prior art.The smaller area requirement allows multiple devices to be fabricated ona single I.C.

FIGS. 11-14 illustrate in plan views the metallization pattern for anexample seven transistor I.C. using lateral DMOS devices. The advantagesof the invention enable the production of this I.C. by enabling a singleLDMOS power transistor of a desired performance characteristic to befabricated in a small enough area of silicon that several such devicescan be fabricated on a single I.C. Without the third level of thickcopper coupling the diffusions of the devices together, and thuslowering the area required for each device, this level of integrationwould not be possible.

FIG. 14 depicts the metallization pattern for the second level of metalfor a seven transistor integrated circuit. The sources of each of theseven transistors 71, 81, 91, 101, 111, 121, and 131 are coupledtogether using common source buss 141. The gate connections are made atgate pads 73, 83, 93, 103, 113, 123 and 133, one for each of the seventransistors. The drain connections are made at drain pads 72, 82, 92,102, 112, 122, and 132, one for each of the seven transistors.

Each transistor 71, 81, 91, 101, 11, 121 and 131 is produced using thestriped source and drain diffusion scheme of FIG. 8. The second levelmetal is used to interconnect the first level of metal, which is notshown here for clarity. The first level metal forms rows of stripeswhich are underneath the second metal and therefore not visible. Thefirst level metal stripes are perpendicular to the second level metalcolumns. The second level metal forms columns of alternating source anddrain busses, one source and one drain buss for each of the transistors.For example, transistor 71 has source bus 72 and drain bus 74. As shownin FIG. 5, the source buss and the drain buss each cover part of each ofthe striped regions of the transistor. Using vias which are also notshown in FIG. 11, the source buss selectively couples the sourcediffusions together by coupling the striped first metal layer regionscovering the source diffusions together. Similarly, the drain bussselectively couples the drain diffusions together. Each source/drainpair of columns in FIG. 11 therefore forms a single LDMOS transistor.Transistor 81 has a source column 84, and a drain column 85. Transistor91 has a source column 94, and a drain column 95. Transistor 101 has asource column 104 and a drain column 105. Transistors 111, 121 and 131are similarly constructed. The drain column for each of the transistorsis coupled to the drain pad. The source columns are all coupled to thesource common buss 141 and to source pads 143. Gate polysilicon runsparallel to the source and drain columns and there is a gate polysiliconcolumn associated with and running between each transistor. The gatepolysilicon columns are coupled to the respective gate pads.

FIG. 12 depicts the mask used to form the vias between the second levelof metal as shown in FIG. 11 and the third level of thick metal, not yetshown. The via pattern shown in FIG. 12 indicates where the protectiveovercoat covering the second level of metal will be opened and coveredwith the third level of metal to form the final interconnected IC. Thebond pads are also shown as having vias in the protective overcoat,however these areas will be bonded to using conventional ball bondingtechniques, so there will be no metal three copper plated onto thoseareas. In FIG. 12, vias are shown formed over each gate bond pad 73, 83,93, 103, 113, 123, and 133, each drain bond pad 72, 82, 92, 102, 112,122 and 132, and over the source pads 143. Also, vias are formed overthe source common buss 141, over the source columns 75, 84, 94, 104,114, 124 and 135, and over the drain columns 74, 85, 95, 105, 115, 125and 134.

FIG. 13 depicts the mask used to pattern the areas to receive the copperplating for the third level of metal. The common source bus 141, thesource columns, the drain columns, and the drain contacting areas, butnot the bond pads, will all be covered with the third level of metal,which will be in physical and electrical contact with the second levelof metal shown in FIG. 11 through the vias as shown in FIG. 12. Allregions of FIG. 13 are numbered identically to FIGS. 11 and 12.

FIG. 14 depicts the composite of FIGS. 11, 12 and 13. The numbering ofFIG. 14 is identical to that of FIGS. 11, 12 and 13. The bond pads areshown as having second level metal, covered with vias in the protectiveovercoat opening a hole for the bonding operations in the pad areasthemselves, and no third metal is on the bond pads. The common sourcebus 141 shows a third metal area over it with an underlying via showncoupling it to a hidden metal two source buss. Each of the sourcecolumns is shown as a metal three region with an underlying via couplingit to a metal two region, which is hidden underneath, all of the sourcecolumns running into the common source buss. Similarly, each of thedrain regions is shown as a contacting area and a columns of metal threewith a via regions lying underneath it coupling it to a metal two draincolumn which is not visible in the figure. A cross section of thecompleted device taken along one of the source columns will be identicalto that shown in FIG. 4.

FIG. 15 is a plot of the results achieved for the seven transistorintegrated circuit design using real data, and comparing the two levelmetal interconnection scheme of the prior art to the three level metalstructure of the preferred embodiment as shown in FIGS. 11-14. Themeasured actual resistance Ron is plotted for each of the seventransistors T1-T7, corresponding to transistors 71, 81, 91, 101, 111,121 and 131 in FIGS. 14-17. The top curve, labeled "a", and using smalltriangles as data points, is for a seven transistor I.C. using the twolevel metallization techniques of the prior art. The bottom curve,labeled "b", and using round circles as data points, is for the seventransistor I.C. of the preferred embodiment incorporating 35 micronthick third level metal as shorting busses, as shown in FIGS. 11-14.

The designed-for Ron expected for the part is about 350 milliohms, basedon its active area. However, actual measured results using the two layermetal conventional interconnection scheme of the prior art result in ameasured Ron of a range of about 480-570 milliohms. FIG. 15 depicts theresults as a function of the distance from the pads. Each transistor'sindividual Ron is shown. The ideal Ron plot would be fiat across theseven transistors. In the prior art curve, labeled "a", it can be seenthat as the transistor location varies, the Rdson varies, showing asignificant debiasing effect. With the conventional metallizationinterconnect scheme, the Ron increases and the variance across the partis due to current debiasing, which also decreases the Vgs of thetransistor and results in a lower drive and thus a lower safe operatingarea rating.

In contrast, the second curve plotted in FIG. 15 depicts the measuredresults obtained when thick copper third level metal layer is added,overlying the source bus and the drain busses over the columns of secondlevel metal. This third layer of thick level metal provides a greatreduction in the overall resistance of the metallization structure seenfrom the pads, which results in lower overall Ron and improvedperformance. This is shown by the flatter curve for Ron for thepreferred embodiment as measured and plotted in FIG. 15. Because thereis less debiasing and less variance from transistor to transistor, thegate to source voltage Vgs remains more ideal, drive for the transistorremains high, and a higher safe operating area rating is obtained forthe same silicon area because of the uniformity of operation of thedevice.

In FIG. 15 the transistor T1 is farthest from the source pad 143 and ingeneral, because of source buss debiasing, the transistor farthest fromthe source pads will have the highest active on resistance Ron. Thelocations of the transistor T1 and the source pads 143 can be seen bestin FIGS. 11-14. The plot depicted in FIG. 15 not only shows that the Ronobtained for T1 using the conventional prior art two level metallizationscheme is higher than the Ron obtained using the three level thick metalcopper scheme of the preferred embodiment, but FIG. 15 also shows thatthe distribution of Ron obtained from transistor T7 to transistor T1 isvery poor in the conventional two level metal case, with a variance of90 milliohms. In contrast, the plot for the transistors T1-T7 using thepreferred embodiment shows Ron for each of the seven transistorsfabricated using the three level metal with the thick copper third levelto be about 350 milliohms, with a fairly fiat distribution. This resultsin an integrated circuit having a plurality of power transistors withuniform operation. If the user connects the transistors to form acircuit, such as an H bridge, each of the devices will operate uniformlyproviding excellent overall circuit performance.

The exemplary LDMOS devices described above provide an excellentapplication for the invention described herein. Other laterally formeddevices are also compatible with the third level metal shorting bussesof the invention. For example, a lateral zener diode may be formed andused as an ESD protection circuit. The zener diode has p and ndiffusions formed in diffusion wells, the p and n diffusions alternatingand forming stripes of anode and cathode material. These striped regionsare then each covered with first metal material and coupled togetherusing a metallization scheme similar to the LDMOS transistors above. Thesecond metal busses are used to couple the anode and cathode regionstogether and to the other diodes.

As for the LDMOS transistor case, the resistance of the diodes used forESD protection is critical to device performance. FIG. 16 depicts acombined cross section and schematic view of a stacked diode ESDprotection circuit which protects the active circuitry of an I.C. fromESD strikes by breaking down zener diodes 235, 237 and 239. The activecircuitry will typically be LDMOS or MOS transistors which have thingate oxides and parasitics that are susceptible to breakdown when highvoltages are applied. The three diode stack is coupled to the gateterminal Vg of the devices to be protected. If the overall resistance ofthe diodes forming the protection circuitry is high, the breakdown timeis increased. This time is similar to a switching speed for atransistor. The breakdown speed is inversely related to the diodejunction capacitance and the resistance. As the resistance is lowered,the breakdown speed is enhanced and the protection offered by the ESDdiode stack 235, 237 and 239 of FIG. 16 improves, because it is muchless likely that the active circuitry will be damaged before the zenerstack breaks down and routes the electrostatic discharge strike currentaway from the active circuitry.

FIG. 16 depicts, in cross section, an exemplary three diode stack to beused in the protection circuitry of FIG. 16. Each diode 235, 237 and 239is a lateral diode which is formed in a manner that is compatible withLDMOS processing. The diffusion wells 209 are formed by a firstdiffusion step into the epitaxial layers 203, and the diodes are formedof alternating stripes of cathode 213 and anode 211 material in a seconddiffusion step. Although in FIG. 16 each diode is shown with only asingle cathode stripe and a single anode striped diffusion, this is donefor clarity only. Each diode has multiple alternating cathode and anodediffusion stripes 213 and 211 diffused into the p diffusion wells 209.Each diode is isolated by the use of a N tank 202 formed underneath thep diffusion wells. Parasitic diodes 240 are used to prevent pnp lateralbipolar parasitics from becoming active and conducting unwanted current.

In operation, when an ESD voltage strike appears at the gate input Vg,the diodes 237 and 239 are reverse biased. If the voltage strike exceedsthe combined breakdown voltage of the diodes 237 and 239, the diodeswill go into zener breakdown. More diodes can be added if needed toprotect the thin gate oxides of a particular device. In a typicalapplication for the circuit of FIG. 16, the LDMOS transistor beingprotected has a gate oxide rupture voltage of about 30 volts. Using anexisting LDMOS process, the zener diodes breakdown between 8 and 9volts, and a forward biased voltage drop of about 0.7 volts. The zenersmust not breakdown for normal input voltages, and for this applicationthe maximum normal input voltage is about 15 volts, so the three diodeconfiguration is used. The breakdown voltage, which is the combinationof the zener breakdown voltages plus a forward biased diode drop, is16-19 volts, which exceeds the normal operating voltage but is less thanthe gate oxide rupture voltage. For an ESD strike at the gate input,diode 235 will be forward biased, and the gate input will be voltageclamped to the voltage of the diode stack. This clamping action willprevent damage to the gate oxides of the active circuitry. Diode 235 isreverse biased when the gate terminal Vg goes negative, and operates inthe same manner to prevent a negative strike from harming the gateoxides of the active circuitry.

FIG. 17 depicts, in cross section, a detailed view of one of the diodesof FIG. 16 and depicts the second and third level metallizationstructure of the preferred embodiment. In FIG. 17, a portion of the pdiffusion well 209 is shown with multiple cathode diffusions 213 andmultiple anode diffusions 211. N buried layer 201 is formed oversubstrate 202, as the bottom of an N tank. Deep diffusions 207 and n+contacting regions 221 complete the tank. First metal stripes 227contact the cathode diffusions 213. First metal stripes 215 contact theanode regions 211. Oxide 223 isolates the first metal from thediffusions, and vias are patterned over the contacting regions. Oxide244 is used to isolate the first metal. Vias are patterned in oxide 244and the second metal layer 245 is formed, contacting the first metal inthe anode regions, in this case, so that second metal layer 245 is ananode second metal buss running over the striped regions andperpendicular to them. Third metal shorting buss 253 is a copper busswhich is formed as described above. The protective overcoat is formedover the second metal layers and then stripped away where it is desiredthe third metal layer contact the second metal layer, as in FIG. 17. Abarrier layer and the copper seed layer are deposited, photoresist isused to pattern the copper layer and then the thick copper buss 253 isplated onto the seed layer.

FIG. 18 depicts a plan view of the three diode circuit of FIG. 16 andincorporates the third level thick copper shorting busses of theinvention. In FIG. 18, the three diode tanks of FIG. 16 are shown havingfirst metal covering the striped geometry diode cathode and anodediffusions. Second metal busses 43 and 45 couple the cathode and anoderegions together by selectively coupling to either anode or cathodestriped regions. The individual diode cathodes and anodes are coupled toform the three diode stack of FIG. 16, diodes 235, 237 and 239. Aprotective overcoat, not visible in the drawing, is formed over theentire structure using conventional techniques. Via regions 251 areformed in the protective overcoat where the third level metal shortingbuss is to contact the second level of metal. The third level copper 253is plated onto these areas in the same manner as described above withrespect to the LDMOS transistors and with also described above withrespect to FIG. 17.

In operation, the overall resistance of the diode stack of FIG. 18 isgreatly reduced and performance is improved over the prior art due tothe lowered resistance which results from the use of the third levelmetal of thick copper. The breakdown speed is increased which enhancesthe ability of the diodes to protect the active circuits in the case ofan ESD strike. A high performance ESD circuit can be fabricated in lessarea than before because the resistance of the structure including thethird level of metal is far less then the resistance obtained using theprior art metallization structures. The third level of thick metal maybe used to lower resistance in other circuit devices as well, such aslateral resistors, capacitors, diode connected transistors and capacitorconnected transistors, etc.

While this invention has been described with reference to illustrativeembodiments, this description is not intended to be construed in alimiting sense. Various modifications and combinations of theillustrative embodiments, as well as other embodiments of the invention,will be apparent to persons skilled in the art upon reference to thedescription. It is therefore intended that the appended claims encompassany such modifications or embodiments.

What is claimed is:
 1. An ESD protection circuit for LDMOS devices,comprising:a gate input for coupling to the gate terminal of an LDMOStransistor; a reference voltage input for coupling to a referencevoltage; and a plurality of zener diodes serially coupled between saidgate input and said reference voltage input, each one of said zenerdiodes comprising:a diffusion well diffused into a semiconductorsubstrate; a plurality of cathode diffusion regions of a firstsemiconductivity type; a plurality of anode diffusion regions of asecond semiconductivity type, spaced apart from an interdigitated withsaid cathode diffusions so that said anode and cathode diffusionsalternate; a plurality of first metal cathode stripes, each overlyingand in electrical contact with a respective one of said cathodediffusions; a plurality of first metal anode stripes, each overlying andin electrical contact with a respective one of said anode diffusions; asecond metal anode buss partially overlying each of said first metalanode stripes and each of said first metal cathode stripes, andselectively electrically contacting said first metal anode stripes; asecond metal cathode buss spaced apart from said second metal anodebuss, said second metal cathode buss partially overlying each of saidfirst metal anode stripes and each of said first metal cathode stripes,and selectively electrically contacting said first metal cathodestripes; a thick copper third level anode shorting buss partiallyoverlying and in electrical contact with said second metal anode buss,said thick copper third level anode shorting buss substantially loweringthe resistance of the diode anode; and a thick copper third levelcathode shorting buss partially overlying and in electrical contact withsaid second metal cathode buss, said thick copper third level cathodeshorting buss substantially lowering the resistance of the diodecathode.
 2. A method for providing a ESD protection for an LDMOStransistor, comprising the steps of:providing a gate input coupled tothe gate terminal for the LDMOS transistor; providing a voltagereference input coupled to a reference voltage; and providing aplurality of zener diodes serially coupled between said gate input andsaid reference voltage by performing the steps of:providing a diffusionwell diffused into a semiconductor substrate; providing a plurality ofcathode diffusions diffused into said diffusion well; providing aplurality of anode diffusions diffused into said diffusion well, saidanode and cathode diffusions being spaced apart and alternating;providing a plurality of first metal anode stripes, each one overlyingand in electrical contact with a respective one of said anodediffusions; providing a plurality of first metal cathode stripes, eachone overlying and in electrical contact with a respective one of saidcathode diffusions; providing a second metal anode buss partiallyoverlying each of said first metal anode stripes and said first metalcathode stripes, said second metal anode buss electrically contactingsaid first metal anode stripes; providing a second metal cathode busspartially overlying each of said first metal cathode stripes and saidfirst metal anode stripes, said second metal cathode buss electricallycontacting said first metal cathode stripes; providing a copper thirdlevel anode shorting buss partially overlying and electricallycontacting said second metal anode buss, said thick copper third levelanode shorting buss substantially lowering the resistance of said diodeanode; and providing a copper third level cathode shorting busspartially overlying and electrically contacting said second metalcathode buss, said copper third level cathode shorting busssubstantially lowering the resistance of said diode cathode.